This application claims the benefits of Korean Patent Application Nos. 2003-28395 and 2004-26209, filed on May 3, 2003 and Apr. 16, 2004, respectively, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a proton conducting polymer, and more particularly, to a proton conducting polymer that does not cause water flooding, has good high-temperature properties and can be prepared at a low cost, a polymer membrane comprising the polymer, a method of manufacturing the polymer membrane, and a fuel cell manufactured using the polymer membrane.
2. Description of the Related Art
Recently, with growing concerns about the environment and the exhaustion of energy resources, and the commercialization of fuel cell automobiles, there is an urgent need for the development of reliable, high-performance fuel cells that are operable at an ambient temperature with high-energy efficiency and for the development of polymer membranes capable of increasing the efficiency of fuel cells.
Fuel cells are power generating systems that convert energy produced through the electrochemical reactions of fuel and oxidative gas directly into electric energy. Such fuel cells can be categorized into electrolyte fuel cells containing molten carbonate salt, which are operable at a high temperature of 500-700° C., electrolyte fuel cells containing phosphoric acid, which are operable around 200° C., and alkaline electrolyte fuel cells and polymer electrolyte fuel cells operable between room temperature and 100° C.
The polymer electrolyte fuel cells include proton exchange membrane fuel cells (PEMFCs) using hydrogen gas as a fuel source and direct methanol fuel cells (DMFCs) which generate power using liquid methanol directly applied to the anode as a fuel source. The polymer electrolyte fuel cells, which are emerging as a next generation clean energy source alternative to fossil fuels, have high power density and high-energy conversion efficiency. In addition, the polymer electrolyte fuel cells work at an ambient temperature and are easy to hermetically seal and miniaturize, so they can be extensively applied to the fields of zero emission vehicles, power generating systems for houses use, mobile telecommunications equipment, medical equipment, military equipment, space equipment, and the like.
The basic structure of a PEMFC as a power generator producing a direct current through the electrochemical reaction of hydrogen and oxygen is shown in FIG. 1.
Referring to FIG. 1, a PEMFC includes a proton-exchange membrane 11 interposed between an anode and a cathode.
The proton-exchange membrane 11 is composed of a solid polymer electrolyte with a thickness of 50-200 μm. The anode and cathode, respectively, include anode and cathode backing layers 14 and 15 for supplying reaction gases, and catalyst layers 12 and 13, in which oxidation/reduction of reaction gases occur, forming gas diffusion electrodes (hereinafter, the anode and cathode will be referred to as “gas diffusion electrodes”). In FIG. 1, a carbon sheet 16 has gas injection holes and acts as a current collector.
As hydrogen, a reactant gas, is supplied to the PEMFC, hydrogen molecules decompose into protons and electrons through an oxidation reaction in the anode. These protons reach the cathode via the proton-exchange membrane 11. Meanwhile, in the cathode, oxygen molecules take the electrons from the anode and are reduced to oxygen ions. These oxygen ions react with the protons from the anode to produce water.
As shown in FIG. 1, in the gas diffusion electrodes of the PEMFC, the catalyst layers 12 and 13 are formed on the anode and cathode backing layers 14 and 15, respectively. The anode and cathode backing layers 14 and 15 are composed of carbon cloth or carbon paper. The surfaces of the anode and cathode backing layers 14 and 15 are treated so that reaction gases and water can easily permeate into the proton-exchange membrane 11 before and after reaction. Hydrogen in the PEMFC is obtained by modifying natural gas or methanol. In the modification, carbon monoxide, which poisons a Pt catalyst, is generated and the performance of the fuel cell is considerably lowered. To increase a poisoning resistance to carbon monoxide, it is necessary that cell reaction only occurs at temperature of at least 100° C.
A proton conducting polymer membranes are used as a proton exchange membrane interposed between the anode and the cathode of the PEMFC. Polymers used for proton-conduction polymer membranes require high ionic conductivity, electrochemical stability, acceptable mechanical properties, thermal stability at working temperatures, the possibility of being processed into low-resistant thin films, small degree of swelling when soaking up liquid, etc. Fluorine-based membranes having fluorinated alkylene in their backbone and sulfonic acid groups at the terminals of fluorinated vinylether side chains, such as Nafion by Dupont, are currently available as proton conducting membranes. However, such fluorine-based membranes are unsuitable for automobile fuel cells due to their high price caused by a complicated substitution process with fluorine. Also, due to low moisture content, moisture in the fluorine-based membranes is vaporized when driving the fuel cells at a temperature of 100° C. or higher in order to prevent a catalyst from poisoning. As a result, the ionic conductivity is sharply decreased and driving of the fuel cell stops. Also, when driving the fuel cells at 100° C. or less, water flooding in the cathode occurs at a high current density. Accordingly, the effective area of the electrode reduces and the efficiency of the fuel cell deteriorates.